Modulator distortion correction



Oct. 28, 1947.

H. ROMANDER MODULATORDISTORTION CORRECTION Filed April 10, 1942INVENTOR. fl/sa fi'a/m/v a? ATTORNEY.

AUD/ 0 Patented Oct. 28, 1947 MODULATOR DISTORTION CORRECTION HugoRomander, North Caldwell, N. J assignor to Federal Telegraph Company,Newark, N. J a corporation of California I Application April 10, 1942,Serial No. 438,431

2 Claims.

This invention relates to modulating systems In systems for modulatingcarrier waves with appreciable power, difficulty is often experienced inobtaining a suitably close agreement between the modulating or signalWave and the resulting envelope of the modulated carrier wave. This lackof agreement may be due to distortion of the modulating wave, or to theintroduction into the modulated wave envelope of a hum or otherinterfering component as from the alternating current supplying energyto the cathode heaters or other parts of the modulating system, or maybe due to other causes. Modulating systems have heretofore been proposedfor correcting for undesired modulation of the modulated wave byintroducing an audio frequency correcting wave into the audio frequencymodulating circuit, however this expedient tends objectionably tointerfere with the normal construction and operation of the audiofrequency modulating circuits and thus leads to further difficulties.

An object of the present invention is to counteract or avoid thedifficulties referred to above.

Another object is to correct or compensate effectively and in a simplemanner for the distortion or hum in the modulated wave envelope, or forother undesired failure of the envelope to agree precisely with the waveform of the original modulating wave.

Another object is to accomplish one or more of the objectives referredto above without altering the construction or operation of the originalaudio frequency modulating wave circuit path in the modulating system.

In accordance with the present invention a method of operation andcircuits are provided for attaining the above objectives. These andother objects and features of the invention will be understood from thefollowing detailed description in connection with the accompanyingdrawing of examples illustrative of the invention.

In the drawing, Figs. 1 and 2 show circuit diagrams of modulatingsystems adapted for carrying out the invention.

In Fig. 1, the modulating system includes the carrier wave source lhaving a modulator-amby adjustment of the variable tuning capacitor 3 inshunt with the inductor 4, constituting together a resonant frequencydetermining circuit for the oscillator tube 5 of the electron dischargetype having an anode, a cathode and a grid. The modulator-amplifier 2,includes the tube 6 of the electron discharge type having an anode, acathode and a grid. The output or tank circuit 1, 8, tuned to resonancewith the carrier wave from source I, includes the variable tuningcapacitor l in shunt with the output inductor B, the cathode of tube 5being connected by way of the low impedance condenser 9 to anintermediate point of inductor 8 so that neutralization of the amplifiermay be obtained by adjustment of the variable capacitor Ill connectingthe grid of the tube in well-known manner with the end of inductor 8remote from the anode of the tube. Inductor 8 constitutes the primary ofthe output transformer of the modulating system, the secondary ll ofwhich is connected with an antenna or other suitable load circuit.

Direct current energy is supplied to the carrier wave source I over apath which may be traced from the ground through the direct currentsource I2, and conductor l3 to the anode of tube 5, the cathode of thetube being grounded. The carrier wave from source l is transmitted fromthe anode of tube 5 to the grid of tube 5 through the capacitor M of lowimpedance to the carrier frequency but of high impedance to waves ofmodulating frequency. Capacitor I4 together with the grid-cathode leakresistance l5 constitute a capacity-resistance coupling between tubes 5and 6. The direct current energy supply circuit for tube 6 may be tracedfrom the ground through the direct current source Hi, conductor I1, andinductor 8 to the anode of the tube, the

cathode of the tube being grounded.

plifier Ecoupled therewith. The carrier wave A wave of audio or othermodulating frequency which is of relatively lower frequency than thecarrier wave, is impressed on the primary of input transformer Hi, thesecondary of which is in the input of the balanced amplifier l9, havingan output inductor constituting the primary of transformer 29, thesecondary of which is in: cluded in conductor l'l supplying directcurrent energy to the anode of tube 6, The conductor I! thus serves as amodulating wave input for tube 6. Carrier frequency choke coil 2| ofhigh impedance to waves of carrier frequency and of lowimpedance toWaves of modulating frequency,-

prevents Waves in the carrier frequency range in modulator-amplifier 2from reaching the transformer 20, 1;

A secondary inductor 22 is magnetically coupled with the output inductor8 of modulatoramplifier 2 and has the variable capacitor 23 in shunttherewith for tuning circuit 22, 23 to rescnance with the carrier wavefrom source I. In series with inductor 22 is connected the demod ulator24 consisting of a diode rectifier tube of the electron discharge type.The capacitor 25 in series between rectifier 24 and inductor 22 is oflow impedance to waves in the carrier fre-- quency range and of highimpedance to waves of modulating frequency. sistance 26, in shunt withcapacitor 25', pro-. vides a path for the demodulated wave of modulatingfrequency. Another potentiometer re-v sistance 2'! is connectedeffectively across the secondary of transformer 28 to provide anadjustable component of the original modulating wave.

A blocking capacitor 28 prevents the passage of direct current fromresistance 2'! to the secondary of transformer 29 and prevents the highpotential from source Hi from reaching resistance 21.

In accordance with the normal operation of the modulating system, acarrier wave from source I is transmitted to modulator-amplifier 2 whichproduces amplified carrier waves in output inductor 8. Modulating wavestransmitted through transformer l8 and the input amplifier [9 areimpressed upon the energy supply conductor Ii serving as a modulatingWave input for modulator-amplifier 2, thus causing themodulator-amplifier to produce an output wave in the tank circuit 1, 8,consisting of the amplified carrier wave modulated substantially inaccordance with the modulating wave.

Any distortion, alternating current power hum as that due to alternatingcurrent heating (not shown) of the cathodes or any other undesiredcomponent introduced by tubes 5 or 6 or their connected circuits,causing the envelope 0fv the modulated wave in tank circuit 1, 8 todiffer from The potentiometer rethe wave form of the original modulatingwave,

is reproduced in the demodulated wave in potentiometer 26. In order toprovide a correction for this undesired distortion, hum, or otherinterference, a correction circuit conductor 36 is provided whichincludes a variable portion of each ofthe potentiometers 26 and 2'!connected in series with each other and in phase opposition with respectto the instantaneous values, of the modulating and demodulated waves.Conductor 30 connects with the grid of the first stage of amplifier Mwhich serves to amplify the correcting wave and Supply an amplifiedcorrecting wave through the primary of transformer 32-, the secondary ofwhich is included in conductor l3 supplying direct current energytothecarrier wave source I.

Amplifier 3! may be of any well-known form being preferably adapted toamplify without ap preciable rotation of the phase'of components of thecorrection wave. A choke coil 33 in conductor i3 is of low impedance tothe correction wave and of high impedance tothe wave of carrierfrequency, so that the carrier wave is isolated from transformer 32. Byupward adjustmentof the sliding contact onpotentiometers 25 and 2?,

respectively, the amplitude of the demodulating V wave and themodulating wave, respectively, in circuit 39, may be increased to anysuitable value, while downward adjustment of the sliding con tactsreduces the respective amplitudes of these waves.

Potentiometers 26 and 21 are preferably so adjusted that only thedifference component representing the difference in wave form betweenthe original modulating wave and the envelope of the modulated carrierwave, is transmitted in amplified form to the anode of tube 5. Thecorrection wave, is applied to source I in that one of the two. possiblephases degrees from each other which opposes the distortion, hum orother interfering component appearing in the envelope of the modulatedwave in the output inductor 8 of the modulator-amplifier 2. Theamplitude of the correction wave is adjusted by potentiometers 26 and,21 to a suitable value for efficiently correcting for the undesiredmodulation.

The negative polarizing source 34 is provided in the circuit30. for thegrid of the first stage of amplifier 3|.

The circuit of Fig. 2 is similar in its general arrangement andoperation to that of Fig. 1, differing therefrom in having a directconnection between the rectifier 24 and the anode of modulator-amplifier2 the rectifier being effectively in shunt between the cathode and anodeof tube 6 and the tank circuit l, 8, thus avoiding the necessity of aresonant circuit such as circuit 22, 23 in Fig. l, inductively coupledwith the tank circuit 1', 8. The necessity of providing potentiometer 21in Fig, 1 is, also, avoided since it will be seen that the modulatingwave potential is present on the anode or modulator-amplifier 2. It willbe seen that the full carrier wave potential of the anode of tube 6 isapplied to the cathode of rectifier 24. In this form of circuit, it isnecessary to make the direct current potential of the anode circuit ofmodulator-amplifier 2, exactly equal to the peak carrier wavevoltageappearing on the anode; in order to obtain the desired ad'-justrnent of the circuit. Under these conditions thepeak of the negativehalf cycle of each cycle of the carrier wave causes the anode current toreduceto zero value. A direct current polarizing source 35, having apotentiometer 36 connected in shunt therewith, is provided in serieswith rectifier 24 in order to produce a suitable value of space currentthrough the rectifier at those instants when the negative carrier wavepeaks are substantially equal and opposite to the average direct currentanode potential of tube 6. This meansthatthe potential of theanode oftube '6 will be positive-R all times and will vary between zero. andtwice thepotential supplied by conductcr [1, thus preventing tube 24from rectifying except for a small portion of the carrier wave cyclewhen the anode of tube 6 is less positive than the'anode of tube 24. Itwill be seen that a small amount of current passes through rectifier 24at each negative peak of the carrier wave and will vary in amplitude inaccordance with the difference of wave form between the originalmodulating wave and the corresponding envelope of the modulated wave,thus producing a correction wave corresponding with that obtainedin theFig. 1 circuit.

According to a preferred operation of the circuits of Figs. 1 and 2, thecarrier wave voltage "amplitude applied to the grid ofmodulatoramplifier tube 6 should be of large value, but not so large asto produce saturation of output, that not so large as to produce themaximum grid-driven output oscillations in tank circuit 78. The gridexcitation is preferably of such value as to permit the slightvariations of amplitude of the arri-er wave, representing the correctionwave as applied to the grid, to produce the necessary correctingcomponent in the tank circuit to oppose and thus neutralize the effectof the undesired modulation component therein. Moreover, in order toenhance the preferred operation referred to above, it is desirable thatthe grid of tube 6, receive a component of negative bias potential froma constant voltage source 29, such as a battery, in addition to or inplace of any negative bias potential derived from capacitor M inconjunction with resistor I5, thus making it possible to obtain readilythe desired operating point on the grid-potential anode-currentcharacteristic of tube 6 without appreciable reduction in efficiency ofthe modulator-amplifier.

Capacitor 25, as in Fig. 1, provides a path for carrier wave current.The conductor 31 in shunt with capacitor 25 includes the primary oftransformer 38 adapted to transmit the demodulated waves in modulatingfrequency range, representing the difference between the modulating waveimpressed on the anode of tube 6 by transformer 20 and the demodulatedwave produced by rectifier 24. The carrier wave choke coil 39 in serieswith the primary of transformer 38 prevents the carrier wave from beingtransmitted to the transformer. As in the case of Fig. 1, the amplifierfor transmitting the correction wave to the anode of oscillator 5 may beof any suitable form, the amplifier 49 in Fig. 2 being shown as atransformer coupled multiple stage amplifier, but may be of the sameresistance coupled form as that shown for amplifier 3| in Fig. 1.

While the circuit of Fig. 2 is somewhat simpler in certain respects fromthat of Fig. 1, it will be seen that in Fig. 2, the cathode of rectifier24 is subjected to the high voltage which may be present on the anode oftube 6. It is therefore necessary to provide more insulation in Fig. 2and to avoid capacity effects incident to the rectifier circuitsconnected directly with the anode of tube 6.

It will be noted that in the circuits of Fig. 1 and Fig. 2 thecorrection wave is applied to the energy supply circuit for the carrierwave source I over a path I3 entirely isolated from and independent ofthe path of conductor I! for the original modulating wave. Thecorrecting wave, therefore, produces no disturbing effect either on themodulating wave input transformer 20 or the path for the modulating wavethrough conductor Number ing excitation of the input of the cascadestages I, 2 by a correcting wave of relatively large amplitude of thekind employed with a negative feed back, The normal operation of thetube circuits is thus maintained with minimum interference from theaum'liary correcting circuits. A further advantage of the presentarrangement is that full use may be made of the maximum amplification oftubes 5, 9, or additional stages of amplification (not shown), withoutreduction of amplification of the type involved in the use of negativefeed back circuits.

Various modifications of the method and apparatus disclosed herein maybe made without departing from the invention in its broader scope. Whilethe modulating system disclosed by way of example in connection with theinvention is of the type employing a carrier wave source coupled with amodulator-amplifier, it will be understood that the invention isapplicable, also, to other types of modulating systems. The novelfeatures of the invention are set forth in the appended claims.

What I claim is:

1. A modulating system comprising a modulator-amplifier having an inputcircuit, a carrier wave source having an output circuit coupled to saidinput circuit, a rectifier having a cathode coupled to and varying inpotential with that of said output circuit, means for maintaining therectifier anode at a potential substantially the same as the peakcarrier wave potential in said output circuit in the absence ofdistortion so as to demodulate the portion of said carrier wave peak dueto distortion, and means for modulating the output wave of said carrierwave source in accordance with the rectifier output wave.

2. A modulating system as set forth in claim 1, in which the carrierwave source includes a vacuum tube, and the means for modulating theoutput wave of said carrier wave source include means for varying theenergy supplied to the anode of said tube.

HUGO ROMANDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Finch Feb. 25, 1936 Fyler May 4, 1937Craft Dec. 12, 1939 Lunnon et al. June v8, 1936 Brown July 27, 1943Mouromtsefi et a1. Sept. 20, 1938 FOREIGN PATENTS Country Date GermanyOct. 19, 1937

